Infant cups on the market use standard helical threads for closure of a lid portion to a cup portion. As the majority of cups are made of plastic, there are several disadvantages when using this type of closure feature. Typical materials used in this category include polyolefin based materials, such as polypropylene or polyethylene. When molded, these materials tend to bind when torqued against each other during closure. This is the type of action that can occur using helical threads as the closure mechanism. This binding often causes lids that have been connected to the cup using helical threads to be difficult to remove. Also, these polyolefin based materials have a high degree of variability that is further increased by using different color polyolefin in the production of the cup and lid. This variability makes it difficult to ensure proper function of the complex geometry used in making a lid and cup combination so that the container and lid close sufficiently to ensure complete or near complete resistance to leakage.
Also, to ensure that there are no in-mold stresses during normal wear of the product requires that “best practices” for the plastic part design is to maintain constant wall stock thickness where possible. Deviations in thickness in the main wall stock of the container and/or lid adds to the possibility of in-mold stresses and, concomitantly, potential deformation of the part. Deformation of the part may, in turn, cause leakage of liquid contained in the cup.
Further, helical threads also often cause inconsistent wall stock thickness on both the male and female mating parts. Moreover, helical threads in plastic parts tend to be difficult to align at the beginning of the threading process that sometimes leads to damage of the threads on either or both of the lid or the cup. Such damage can lead to improper sealing that again could lead to unwanted leakage.
So-called “sippy cup” lids are generally non-symmetrical parts, mostly due to the addition of a non-symmetrical drinking feature, such as a spout in a raised position on and above the upper surface of the lid. The use of non-symmetrical parts makes it difficult to achieve a plug seal when mating the lid and cup. This shortcoming is compounded by having uneven wall stock in the threads, again usually due to the material and manufacturing processes employed in producing the threads on the lid and the cup. These two features, difficulty in achieving a plug seal and uneven wall thickness, often lead to the parts having the tendency to be “out of round” or not flat, depending on the portion of the lid and cup combination in question. In order to achieve a good, preferably leak-proof, seal with a part that has the above tendencies, the lid must be “torqued-on” with a high degree of force (on the order of 25 lbs. or greater) in order to flex the seal geometry such that it correctly or completely plugs. This force is greater than most end users can comfortably achieve. Moreover, such repetitive force may have the additional drawback of prematurely wearing the threads on the lid or the cup, or both.
It is desirable to manufacture a container and lid having a closure mechanism that does not utilize threads as well as a method for manufacturing a thread-free closure mechanism for a container assembly.